The largest study of its kind suggests postmenopausal women who drink two or more diet drinks a day may be more likely to have a heart attack or stroke.

According to research conducted by a team at the University of Iowa, women who consumed two or more a day were 30% more likely to suffer a cardiovascular event and 50% more likely to die from related disease than women who never or only rarely consume diet drinks.

"We were interested in this research because there was a relative lack of data about diet drinks and cardiovascular outcomes and mortality," Dr. Ankur Vyas, the lead investigator of the study, said.

For the study, researchers divided a group of women into four consumption groups: two or more diet drinks a day, five to seven diet drinks per week, one to four diet drinks per week, and zero to three diet drinks per month. They were monitored for nearly nine years.

Vyas says the association between diet drinks and cardiovascular problems raises more questions than it answers.

"We only found an association, so we can't say that diet drinks cause these problems," Vyas said, adding that there may be other factors about people who drink more diet drinks that could explain the connection.

"It's too soon to tell people to change their behaviour based on this study; however, based on these and other findings we have a responsibility to do more research to see what is going on and further define the relationship, if one truly exists," he adds. "This could have major public health implications."

About one in five people in the U.S. consume diet drinks on a given day, according to data from the National Health and Nutrition Examination Survey, and studies have found artificially sweetened drinks to be associated with weight gain in adults and teens, and seem to increase the risk of metabolic syndrome, which makes both diabetes and heart disease more likely.